The present invention relates to a device for bypassing a computer system interconnecting two network links when this computer system fails.
It is often desirable to insert, between two systems interconnected through a network link, a transparent control system for various possible purposes. The control system may be used for restricting access, for event logging, for ciphering and deciphering.
FIG. 1 schematically shows an arrangement providing this functionality. A system A normally communicates with a system B over a network link, such as an Ethernet link. A control system C is inserted in the network link between systems A and B. In other words, system A is coupled to a network interface card 10 of system C through a network link ethA, while system B is coupled to a second network interface card 11 of system C through a network link ethB.
System C, usually a work station, is supposed to be transparent for systems A and B as regards the network frames exchanged between them. For this purpose, system C executes software which copies the frames between links ethA and ethB. In fact, it mainly operates as a hub. In some case, however, the software may analyze the frames or even perform operations thereon.
Of course, system C is subject to failure. In case of failure, it is desirable to maintain the link between system A and system B. For this purpose, it is known to use bypass devices which are capable of detecting a hardware failure, for example by achieving CRC checks on the frames, in order to switch the network links to bypass links.
However, such devices are not able to detect a software failure, such as an abnormal program termination or a system hang. Indeed, in this case, the frames do not have errors, they are simply not copied from one link to the other.
An object of the present invention is to provide a bypass device capable of achieving its function in case of a software failure of a control system.
This object and others are achieved by executing on the transparent system a software which provides a predetermined control signal on a control line. The control line is polled by the bypass device. The bypass device will bypass the transparent system when the control signal is not present on the control line.
According to a preferred embodiment of the invention, the control signal is in the form of periodic pulses and a bypassing is caused when a pulse is not received on the control line within a predetermined period of time.
The bypass device includes a switch having a first position selecting a main path between first and second network links, including the transparent system, and a second position selecting a bypass path between the first and second network links. A control circuit selects the first position of the switch when the control signal is received on the control line, and selects the second position of the switch when the control signal is not received on the control line.
The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not of limitation with reference to the accompanying drawings.